Answer:
The story of Jonah is the amazing tale of a disobedient prophet who, upon being swallowed by a whale (or a “great fish” - see below) and vomited upon the shore, reluctantly led the reprobate city of Nineveh to repentance. The biblical account is often criticized by skeptics because of its miraculous content. These miracles include

• A Mediterranean storm, both summoned and dissipated by God (1:4-16).
• A massive fish, appointed by God to swallow the prophet after he was thrown into the sea by his ship’s crew (1:17).
• Jonah’s survival in the belly of the fish for three days and three nights, or his resurrection from the dead after being vomited upon the shore, depending on how you interpret the text (1:17).
• The fish vomiting Jonah upon shore at God’s command (2:10).
• A gourd, appointed by God to grow rapidly in order to provide Jonah with shade (4:6).
• A worm, appointed by God to attack and whither the shady gourd (4:7).
• A scorching wind, summoned by God to discomfort Jonah (4:8).

Critics also find Nineveh’s repentance (3:4-9) hard to believe, though it isn’t technically a miracle. In actual fact, Nineveh’s repentance makes perfect sense given Jonah’s extraordinary arrival upon the shores of the Mediterranean and the prominence of Dagon worship in that particular area of the ancient world. Dagon was a fish-god who enjoyed popularity among the pantheons of Mesopotamia and the eastern Mediterranean coast. He is mentioned several times in the Bible in relation to the Philistines (Judges 16:23-24; 1 Samuel 5:1-7; 1 Chronicles 10:8-12). Images of Dagon have been found in palaces and temples in Nineveh and throughout the region. In some cases he was represented as a man wearing a fish. In others he was part man, part fish—a merman, of sorts.

As for Jonah’s success in Nineveh, Orientalist Henry Clay Trumbull made a valid point when he wrote, “What better heralding, as a divinely sent messenger to Nineveh, could Jonah have had, than to be thrown up out of the mouth of a great fish, in the presence of witnesses, say on the coast of Phoenicia, where the fish-god was a favorite object of worship? Such an incident would have inevitably aroused the mercurial nature of Oriental observers, so that a multitude would be ready to follow the seemingly new avatar of the fish-god, proclaiming the story of his uprising from the sea, as he went on his mission to the city where the fish-god had its very centre of worship” (H. Clay Trumbull, “Jonah in Nineveh.” Journal of Biblical Literature, Vol. 2, No.1, 1892, p. 56).

Some scholars have speculated that Jonah’s appearance, no doubt bleached white from the action of the fish’s digestive acids, would have been of great help to his cause. If such were the case, the Ninevites would have been greeted by a man whose skin, hair and clothes were bleached ghostly white—a man accompanied by a crowd of frenetic followers, many of who claimed to have witnessed him having been vomited upon the shore by a great fish (plus any colorful exaggerations they might have added).

Jonah needed only to cause enough of a stir to gain himself admittance to the king who, upon believing Jonah’s message of imminent doom for himself, would have the power to proclaim a citywide day of fasting and penance. According to the biblical narrative that’s exactly what happened (Jonah 3:6-9). So we see that, given the caveat that Jonah was spewed upon the shore by a great fish, Nineveh’s repentance follows from a very logical progression.

As for Jonah’s aquatic experience (which is the crux of the story), while there is no conclusive historical proof that Jonah was ever swallowed by a fish and lived to tell about it, there is some provocative corroboratory evidence. In the 3rd century B.C., a Babylonian priest/historian named Berosus wrote of a mythical creature named Oannes who, according to Berosus, emerged from the sea to give divine wisdom to men. Scholars generally identify this mysterious fish-man as an avatar of the Babylonian water-god Ea (also known as Enki). The curious thing about Berosus’ account is the name that he used: Oannes.

Berosus wrote in Greek during the Hellenistic Period. Oannes is just a single letter removed from the Greek name Ioannes. Ioannes happens to be one of the two Greek names used interchangeably throughout the Greek New Testament to represent the Hebrew name Yonah (Jonah), which in turn appears to be a moniker for Yohanan (from which we get the English name John). (See John 1:42; 21:15; and Matthew 16:17.) Conversely, both Ioannes and Ionas (the other Greek word for Jonah used in the New Testament) are used interchangeably to represent the Hebrew name Yohanan in the Greek Septuagint, which is the Greek translation of the Hebrew Old Testament. Compare 2 Kings 25:23 and 1 Chronicles 3:24 in the Septuagint with the same passages from the Hebrew Old Testament.

As for the missing “I” in Ioannes, according to Professor Trumbull who claims to have confirmed his information with renowned Assyriologist Dr. Herman V. Hilprecht before writing his own article on the subject, “In the Assyrian inscriptions the J of foreign words becomes I, or disappears altogether; hence Joannes, as the Greek representative of Jona, would appear in Assyrian either as Ioannes or as Oannes” (Trumbull, ibid., p. 58).

Nineveh was Assyrian. What this essentially means is that Berosus wrote of a fish-man named Jonah who emerged from the sea to give divine wisdom to man – a remarkable corroboration of the Hebrew account.

Berosus claimed to have relied upon official Babylonian sources for his information. Nineveh was conquered by the Babylonians under King Nabopolassar in 612 B.C., more than 300 years before Berosus. It is quite conceivable, though speculative, that record of Jonah’s success in Nineveh was preserved in the writings available to Berosus. If so, it appears that Jonah was deified and mythologized over a period of three centuries, first by the Assyrians, who no doubt associated him with their fish-god Dagon, and then by the Babylonians, who appear to have hybridized him with their own water-god, Ea.

In addition to Berosus’ account, Jonah appears elsewhere in the chronicles of Israel as the prophet who predicted Jeroboam II’s military successes against Syria in the 8th century before Christ (2 Kings 14:25). He is said to be the son of Amittai (cf. Jonah 1:1) from the town of Gath-hepher in lower Galilee. Flavius Josephus reiterates these details in his Antiquities of the Jews (chapter 10, paragraph 2). Jonah was not an imaginary figure invented to play the part of a disobedient prophet, swallowed by a fish. He was part of Israel’s prophetic history.

As for the city of Nineveh, it was rediscovered in the 19th century after more than 2,500 years of obscurity. It is now believed to have been the largest city in the world at the time of its demise (see Tertius Chandler's Four Thousand Years of Urban Growth: An Historical Census). According to Sir Austen Henry Layard, who chronicled the rediscovery of Nineveh in his classic Discoveries at Nineveh, the circumference of Greater Nineveh was “exactly three days' journey,” as recorded in Jonah 3:3 (Austen Henry Layard, A Popular Account of Discoveries at Nineveh, J. C. Derby: New York, 1854, p. 314). Prior to its rediscovery, skeptics scoffed at the possibility that so large a city could have existed in the ancient world. In fact, skeptics denied the existence of Nineveh altogether. Its rediscovery in the mid-1800s proved to be a remarkable vindication for the Bible, which mentions Nineveh by name 18 times and dedicates two entire books (Jonah and Nahum) to its fate.

It is interesting to note where the lost city of Nineveh was rediscovered. It was found buried beneath a pair of tells in the vicinity of Mosul in modern-day Iraq. These mounds are known by their local names, Kuyunjik and Nabi Yunus. Nabi Yunus happens to be Arabic for “the Prophet Jonah.” The lost city of Nineveh was found buried beneath an ancient tell named after the Prophet Jonah.

As for the whale, the Bible doesn’t actually specify what sort of marine animal swallowed Jonah. Most people assume that it was a cachalot (also known as the sperm whale). It may very well have been a white shark. The Hebrew phrase used in the Old Testament, gadowl dag, literally means “great fish.” The Greek used in the New Testament is këtos which simply means “sea creature.” There are at least two species of Mediterranean marine life that are known to be able to swallow a man whole. These are the cachalot and the white shark. Both creatures are known to prowl the Mediterranean and have been known to Mediterranean sailors since antiquity. Aristotle described both species in his 4th-century B.C. Historia Animalium.

So we now have three of the four major players: Jonah, Nineveh and the man-eating fish. All that remains is the fourth major player: God. Skeptics scoff at the miracles described in the book of Jonah as if there were no mechanism by which such events could ever occur. That is their bias. We are inclined, however, to believe that there is One who is capable of manipulating natural phenomena in such supernatural ways. We believe that He is the Creator of the natural realm and is not, therefore, circumscribed by it. We call Him “God,” and we believe that He sent Jonah to Nineveh to bring about their repentance.

God has made Himself known throughout history in many diverse ways, not the least of which was His incarnation in the Person of Jesus Christ. Not only does Jesus give us reason to believe that there exists One who is able to perform miracles, He gives us every confidence that such events have, in fact, occurred.

Jesus spoke of Jonah’s ordeal as a real historical event. He used it as a typological metaphor for His own crucifixion and resurrection, itself a miraculous event. Matthew quoted Jesus as saying, “For just as Jonah was three days and three nights in the belly of the sea creature, so will the Son of Man be three days and three nights in the heart of the earth. The men of Nineveh will stand up with this generation at the judgment, and will condemn it because they repented at the preaching of Jonah; and behold, Someone greater than Jonah is here” (Matthew 12:40-41; cf. Luke 11:29-30, 32).

The evidence is such that any Christian should have confidence to believe and any skeptic should think twice before dismissing Jonah as a fairy tale.